Method of detecting nitrogen oxides in gaseous or liquid samples

ABSTRACT

A method for determining nitrogen oxides in gaseous or liquid samples is disclosed. A semi-permeable membrane- type sensor comprising a sensor electrode, reference electrode and an electrolyte having a pH range of 6.8-8.0 or 8.0-9.2 is used. Potential between the electrodes is monitored as a measure of nitrogen oxides in the sample. When an electrolyte having a pH range of 8.0-9.2 is used the electrolyte additionally contains at least 25% v/v of a water miscible solvent having a dielectric constant of at least 10.

BACKGROUND OF THE INVENTION

This application is a division of Ser. No. 038,849, filed Apr. 15, 1987,now U.S. Pat. No. 4,780,185, which in turn was a division of Ser. No.810,412, filed Dec. 18, 1985, now U.S. Pat. No. 4,659,434, which in turnwas a division of Ser. No. 726,957, filed Aug. 26, 1985, now abandoned,which in turn was a continuation of Ser. No. 526,914, filed Aug. 26,1983, now abandoned.

This invention relates to electrochemical analysis and more particularlyto gas detection electrochemical cells.

Gas detection electrochemical cells are known in which a detectionelectrode is arranged in a body containing an electrolyte. A gaspermeable membrane is provided to separate the electrode and electrolytefrom the sample and to permit passage of gas to a film of electroylte,between the membrane and the electrode, which connects the detectionelectrode to a reference electrode.

A number of different gases have been measured using such systems, and avariety of electrolytes have been employed. Frant et al. U.S. Pat. No.3,859,191 describes an HCN-sensing electrochemical cell employing anelectrolyte containing KAg(CN)₂. Riseman et al. U.S. Pat. No. 3,897,315describes an H₂ S-sensing electrochemical cell employing an ion-specificreference electrode. Krueger et al. U.S. Pat. No. 3,803,006 describes anSO₂ -sensing electrochemical cell employing an electrolyte containingNa₂ SO₃. Strickler et al. U.S. Pat. No. 3,649,505 describes an NH₃-sensing electrochemical cell employing an electrolyte containing NH₄C1. Riseman et al. U.S. Pat. No. 3,830,718 describes an NH₃ -sensingelectrochemical cell employing an electrolyte containing ammoniumpicrate. Synnott et al. U.S. Pat. No. 4,105,525 describes an NH₃-sensing electrochemical cell employing an electrolyte containing5,5'-Nitrilodibarbituric acid ammonium salt.

The present invention provides improved electrochemical cells fordetecting any of a variety of chemical gases.

In general the invention features, in a first aspect, an electrochemicalcell for detecting, in a gaseous or liquid sample, any of the gases SO₂.NO_(x) (nitrogen oxide, HNC, NH₃, formic acid, acetic acid, or CO₂. Thecell includes a body and within the body a detection electrode, areference electrode, a liquid electrolyte connecting the electrodes, anda membrane permeable to the gas in close proximity to the detectionelectrode and arranged to separate the electrodes and the electrolytefrom the sample. The electrolyte is buffered to a pH value such that theelectrochemical cell gives a slope of response to the gas within 25% ofa Nernstian response. (As is well known, a Nernstian response is achange in measured potential, with a ten-fold change in concentration,which ideally fits the Nernst equation. For a monovalent ion, thepotential change is 59.16 mv at 25° C.; a response within 25% of this isbetween 44.37 and 73.95 mv). The electrolyte is initially (prior to use)essentially (over 99%, v/v) free of any salt capable of dissociating toform in said electrolyte a weak acid or weak base (other than H₃ O⁺ orOH⁻) which forms upon dissociation of the gas being measured. Thereference electrode in contact with the electrolyte is operative at thepH value of the liquid electrolyte.

In preferred embodiments of the above first aspect of the invention, theliquid electrolyte is an aqueous buffer solution containing, at aconcentration of between 0.5% and 70%, v/v, a water-miscible solventhaving a dielectric constant of at least 10 and being non-interferingwith the detection and reference electrodes.

In other preferred embodiments of the above first aspect of theinvention, the detection electrode is a pH electrode or an ion-specificelectrode selective for an ionic species formed in the electrolyte upondissociation of the gas therein; and the reference electrode is asilver-based electrode, a platinum-based electrode, or an ion selectiveelectrode the primary electrode of which is in contact with theelectrolyte, which electrolyte contains the ionic species for which theion-specific reference electrode is selective. The reference electrodeis selected so that the ionic species for which it is selective issubstantially (over 90%) uncomplexed at the pH of the electrolyte.

In a second aspect of the invention features, in general, anelectrochemical cell for detecting H₂ S in a gaseous or liquid sample,the cell including a body and within the body a detection electrode, areference electrode which is not an ion selective electrode, and amembrane permeable to H₂ S in close proximity to the detection electrodeand arranged to separate the electrodes and the electrolyte from thesample. The liquid electrolyte is an aqueous buffer solution buffered toa pH value such that the electrochemical cell gives a slope of responseto the H₂ S within 25% of a Nernstian response. The reference electrodein contact with the electrolyte is operative at the pH value of theliquid electrolyte.

In preferred embodiments of the above second aspect of the invention,the liquid electrolyte contains a water-miscible solvent having adielectric constant of at least 10 and being non-interfering with thedetection and reference electrodes; the solvent is present in aconcentration of at least 25%, v/v,; the detection electrode is a pHelectrode or sulfide ion selective electrode; the reference electrode isa silver-based electrode or a platinum-based electrode; the referenceelectrode is selected so that the ionic species to which it responds issubstantially (over 90%) uncomplexed at the pH of the electrolyte; andthe electrolyte is initially (prior to use) essentially (over 99%, v/v)free of any salt capable of dissociating in the electrolyte to form HS⁻.

We have discovered relationships in the above systems between the pK ofthe gas being detected and the optimum pH of the liquid electrolyte. Onerelationship is similar for all of the protonated gases, and another issimilar for all of the non-protonated gases. Consequently, in preferredembodiments of all of the above aspects of the invention, where the gasbeing detected is one of the protonated gases H₂ S, HCN, formic acid, oracetic acid, the pH of the liquid electrolyte is within 2.3 units of thepK of the gas, most preferably within 1.0 unit of the pK of the gas.

Where the gas being detected is one of the non-protonated gases SO₂,NH₃, NO_(x), or CO₂, the pH of the liquid electrolyte is at least 1.7units above or below the pK of the gas, most preferably at least 2.7units above or below the pK of the gas.

The electrochemical cells of the invention have the advantages ofstability and economy. For example, the SO₂ electrode is, as far as isknown, the first commercially practical device, prior devices havingcontained salts, dissociating to form HSO₃ ⁻ or SO₃ ^(-`), which wereextremely unstable and tended to decompose during storage. Furthermore,where the electrolyte is essentially free of any salt capable ofdissociating to form the weak acid or weak base which forms upondissociation of the gas being measured, an ion selective electrode canbe used as the detection electrode, an option not available when theelectrolyte already contains the ion to be measured. This advantage canbe particularly important in situations in which two gases with close pKvalues are likely to be present in one sample, so that a pH changeobserved using a pH electrode as the detection electrode could beattributed to either of the gases, and an ion selective detectionelectrode must therefore be used to give an accurate result. A furtheradvantage is that the absence of such a salt can in some cases (e.g. inthe case of HCN detection) allow the avoidance of toxic or corrosivesubstances.

Other objects, features, and advantages of this invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments thereof taken together with theaccompanying drawing, in which:

The FIGURE is a diagrammatic representation of an electrochemical cellembodying the invention.

Referring to the FIGURE, the diagrammatic electrochemical cell,generally designated 1, comprises body 2, detection electrode 3,reference electrode 4, output measuring device 6, liquid electrolyte 5,and selectively permeable surface 7 across which the gas to be detectedpasses. Detection electrode 3 can be a pH electrode or an ion selectiveelectrode which is selective for an ionic species which forms in theelectrolyte upon dissociation of the gas being detected. For example,ion selective detection electrodes advantageously can be used, inaccordance with the invention, to measure H₂ S, HCN, and CO₂. HCN can bemeasured, for example, using an electrode containing a membrane composedof Ag₃ AsS₃ and silver cyanide, as disclosed in copending patentapplication Ser. No. 496,113, filed May 19, 1983, assigned to the sameassignee as the present invention, hereby incorporated by reference.

For measuring gases other than H₂ S, the reference electrode 4 can alsobe an ion selective electrode which is operative at the pH of theelectrolyte. In such case, the electrolyte will contain an ion for whichthe reference electrode can respond. An example of an ion selectivereference electrode suitable for use with the subject gases is a sodiumion selective electrode; its use requires the presence in theelectrolyte of sodium ions, which can be provided by any suitable salt,e.g., NaC1.

The liquid electrolytes of the invention can be used in conjunction withcommercially available gas detecting electrochemical cells, e.g., theSeries 10 electrochemical cells having a pH detection electrode and anAg/AgC1 reference electrode made by HNU Systems, Inc., Newton,Massachusetts, as well as in electrochemical cells employing ionselective detection electrodes, as described above.

The composition of the electrolyte is dependent on the gas to bedetected. Generally, the electrolyte contains an inert buffer and adissociated salt for providing an ion to which the reference electrodecan respond. Optionally the electrolyte also contains up to 70%, v/v, ofa water-miscible solvent having a dielectric constant of at least 10(dielectric constant is a measure of polarity, a necessary property ofthe solvent) and is essentially free of salts of the weak acid or weakbase, other than H₃ O⁺ or OH⁻, formed upon dissociation of the gas beingmeasured.

The pH of the electrolyte is preferably such that the slope of theresponse of the electrochemical cell is within 25% of Nernstian. The pHalso bears a relationship to the pK of the gas, as discussed above; therelationship depends on whether or not the gas is protonated.

The other electrode characteristics, in addition to slope, which varywith electrolyte composition are speed of response, lower limit ofdetection, and stability of response (minimization of drift). Since anelectrolyte composition which optimizes one electrode characteristic maynot optimize others, the composition will normally be tailored tooptimize whatever characteristic is most important for a givenapplication while keeping the other characteristics within acceptablelimits, and while ensuring that the slope is never farther than 25% fromNernstian.

The starting point for the preparation of an electrolyte for a givenapplication generally is to prepare an inert aqueous buffer solution(e.g. phosphate or borate) which has a 0.01 M concentration of a saltcapable of dissociating to form an ion to which the reference electrodecan respond (for example, NaC1 if the reference electrode is Ag/AgC1;NaF if the reference electrode is an ion selective fluoride electrode).A water-miscible, non-interfering solvent having a dielectric constantof at least 10 may then be added, if desired, in a concentration of0.5%, v/v. The pH of multiple samples of the electrolyte is then varied,beginning with a pH value within 2.3 units of the pK of the gas beingdetected, if the gas is protonated, or more than 1.7 units higher orlower than the gas being detected if it is not protonated (the pH of theelectrolyte is lower than the pK in the case of the alkaline gas NH₃,and higher in the case of the other non-protonated gases).

Each sample is then tested in an electrochemical cell and the fourelectrode characteristics discussed above are measured. The pHcorresponding to the best performance is then held constant and thecomposition of the electrolyte is then varied in terms of salt andsolvent concentration to further optimize electrode characteristics.

The water-miscible solvent is an optional ingredient when gasmeasurement is carried out in a liquid in contact with the gas permeablemembrane of the electrochemical cell. When measurement is carried out inthe headspace between the membrane and a liquid sample, or in theatmosphere, or when the gas being measured is SO₂, it is preferable thatthe electrolyte contain a water-miscible solvent.

The reason the solvent is desirable when the membrane is not in contactwith a liquid sample is that, without the solvent, water tends to movein and out of the solution, causing continual drifting of response. Thesolvent minimizes such movement of water and this stabilizes response.When the gas is to be measured in the headspace or the atmosphere, thesolvent is preferably present in a concentration of at least 25%, v/v,and in some instances can be as high as 70%, v/v. Generally, the optimalpH of the electrolyte will increase with an increasing solventconcentration.

The optimal pH of the electrolyte will vary not only with solventconcentration, but with solvent polarity as well. Generally, more polarsolvents require a larger upward adjustment of pH than less polarsolvents.

Any inert, water-miscible solvent of sufficient polarity can be used.Most commonly the solvent is a suitable monohydroxylic alcohol, diol, orpolyol. Examples of useful alcohols are 2-methyoxy ethanol, diethyleneglycol, glycerol, and propylene glycol. Non-alcoholic solvents, e.g.dimethyl sulfoxide, can also be used.

The buffer employed in the electrolyte can be any suitable inert(non-interfering) buffer, e.g. phosphate, borate, formate, citrate,acetate, or Tris.

The reference electrode, as well as being of one of the types mentionedabove, can alternatively be a redox potential electrode, e.g. a platinumelectrode, which does not require the presence in the electrolyte of anyparticular ion; a stable concentration of any electrolyte is sufficient.

The table below shows preferred pH values and preferred pH ranges ofelectrolytes useful for measuring various gases. The table gives pHranges and values for electrolytes containing high and lowwater-miscible solvent concentrations. The table also gives preferredelectrolyte compositions. In each case, the detection electrode isassumed to be a pH electrode and the reference electrode either platinumor Ag/AgC1. Each electrolyte, when used to measure the correspondinglisted gas, gives a slope within 25% of Nernstian.

    __________________________________________________________________________        Weak                                                                          Acid            Preferred             Preferred                               Or       Preferred                                                                            pH Range;                                                                            Preferred pH;  Electrolyte                             Weak     pH Range;                                                                            at Least                                                                             Less Than                                                                             Preferred pH;                                                                        Compositions                        Gas Base   pK                                                                              No Solvent                                                                           25% Solvent                                                                          1% Solvent                                                                            35% Solvent                                                                          Low Solvent                                                                             High                      __________________________________________________________________________                                                        Solvent                   SO.sub.2                                                                          HSO.sub.3.sup.-                                                                      1.8                                                                             4.8-6.0                                                                              6.0-7.2                                                                              5.25    6.5    0.025 M Phos-                                                                           0.05 M Phos-                  or                                    phate Buffer                                                                            phate Buffer                  SO.sub.3.sup.-                        pH 5.25   pH 6.5                                                              0.010 M NaCl                                                                            0.01 M NaCl                                                         0.5% (v/v)                                                                              35% (v/v)                                                           Ethylene Glycol                                                                         Ethylene Glycol                                                               or 0.05 M                                                                     Phosphate                                                                     Buffer; pH 6.5                                                                0.01 M NaCl                                                                   45% (v/v)                                                                     Ethylene Glycol           H.sub.2 S                                                                         HS.sup.-                                                                             7.0                                                                             6.3-7.8                                                                              7.3-8.8                                                                              7.0     8.0    .05 M Phosphate                                                                         0.05 M Borate                                                       Buffer; pH 7.0                                                                          Buffer; pH 8.0                                                      .01 M NaCl                                                                              0.01 M NaCl                                                         0.5% (v/v)                                                                              35% Ethylene                                                        Ethylene Glycol                                                                         Glycol                    HCN CH.sup.-                                                                             9.3                                                                             8.3-9.8                                                                               9.5-11.6                                                                            9.0     10.0   0.1 M Borate                                                                            0.1 M Borate Buffer                                                 Buffer; pH 9.0                                                                          pH 10.0; .01 M NaCl                                                 0.01 M NaCl                                                                             35% Ethylene Glycol       NH.sub.3                                                                          NH.sub.4.sup.-                                                                       9.2                                                                             4.5-6.5                                                                              5.5-7.5                                                                              5.5     6.5    .01 M Phosphate                                                                         .01 M Phosphate                                                     Buffer; pH 5.5                                                                          Buffer; pH 6.5                                                      .01 M NaCl; .5%                                                                         .01 M NaCl; 65%                                                     ethylene glycol                                                                         Ethylene Glycol           NO.sub.x                                                                          NO.sub.2.sup.-                                                                       3.4                                                                             6.8-8.0                                                                              8.0-9.2                                                                              7.25    6.5    .05 M Phosphate                                                                         .01 M Phosphate                                                     Buffer; pH 7.25                                                                         Buffer; pH 8.5                                                      0.01 M NaCl                                                                             .01 M NaCl                                                          0.5% (v/v)                                                                              35% (v/v)                                                           Ethylene Glycol                                                                         Ethylene Glycol           Formic                                                                            HCO.sub.2.sup.-                                                                      3.8                                                                             2.8-4.3                                                                              4.0-5.5                                                                              3.8     4.8    .05 M Phophate                                                                          .05 M Phosphate           Acid                                      Buffer; pH 3.8                                                                          Buffer; pH 4.8                                                      .01 M NaCl                                                                              .01 M NaCl                                                          0.5% (v/v)                                                                              35% (v/v)                                                           Ethylene Glycol                                                                         Ethylene Glycol           Acetic                                                                            CH.sub.3 CO.sub.2.sup.-                                                              4.8                                                                             4.0-5.5                                                                              5.0-6.5                                                                              4.8     5.8    .05 M Phosphate                                                                         .05 M Phosphate           Acid                                      Buffer; pH 4.8                                                                          Buffer; pH 5.8                                                      .01 M NaCl                                                                              .01 M NaCl                                                          0.5% (v/v)                                                                              35% (v/v)                                                           Ethylene Glycol                                                                         Ethylene Glycol           CO.sub.2                                                                          HCO.sub.3.sup.-                                                                      6.4                                                                              9.5-11.0                                                                            10.7-12.0                                                                            9.8     11.0   .01 M Phosphate                                                                         .01 M Phosphate                                                     Buffer; pH 9.8                                                                          Buffer; pH 11.0                                                     .01 M NaCl                                                                              .01 M NaCl                                                          0.5% (v/v)                                                                              35% (v/v)                                                           Ethylene Glycol                                                                         Ethylene                  __________________________________________________________________________                                                        Glycol                

Other embodiments of this invention will occur to those skilled in theart which are within the scope of the following claims.

What is claimed is:
 1. A method for detecting NO_(x) in a gaseous orliquid sample, said method comprising testing said sample in anelectrochemical cell comprising a body and within said body a detectionelectrode, a reference electrode, a liquid electrolyte connecting saidelectrodes, and a membrane permeable to said gas in close proximity tosaid detection electrode and arranged to separate said electrodes andsaid electrolyte from said sample, said liquid electrolyte being anaqueous buffer solution buffered to a pH value of between 8.0 and 9.2and being initially essentially free of any salt capable of dissociatingto form in said electrolyte NO₂ ⁻, said electrolyte containing at least25% v/v of a water miscible solvent having a dielectric constant of atleast 10 and being non-interfering with said detecting and referenceelectrodes.
 2. The method of claim 1 wherein said testing comprisesmeasuring a change in potential between said reference electrode andsaid detection electrode.
 3. The method of claim 1, wherein said testingcomprises measuring a change in potential between said referenceelectrode and said detection electrode.
 4. A method for detecting NO_(x)in a gaseous or liquid sample, said method comprising testing saidsample in an electrochemical cell comprising a body and within said bodya detection electrode, a reference electrode, a liquid electrolyteconnecting said electrodes, and a membrane permeable to said gas inclose proximity to said detection electrode and arranged to separatesaid electrodes and said electrolyte from said sample, said liquidelectrolyte being an aqueous buffer solution buffered to a pH value ofbetween 6.8 and 8.0 and being initially essentially free of any saltcapable of dissociating to form in said electrolyte NO₂ ⁻, said testingcomprising measuring a change in potential between said referenceelectrode and said detection electrode.